If you place items into blast furnaces using hoppers and then go into spectator mode to look into the GUI, the items in the blast furnace will jump into the first 3 slots of your inventory (smelting item in slot 0, fuel in slot 1, output in slot 2) and push the rest of your items across by 3 (slot 5→slot 8, hotbar.8→*poof*).
How to reproduce:
1) Place an item into a blast furnace using a hopper
2) Go into spectator mode
3) Look into the blast furnace
4) You will see the items move into your inventory about 75% of the time
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And I think MC-138592 should just be tagged as "relates to" because this is a different issue that might not be fixed when the new blocks get functionality.
I've also experienced this; it isn't real though. Relogging, opening a chest in survival mode or trying to place it in survival mode will make the items dissapear.